Soil TypesSoils may be separated into three very broad categories: cohesionless, cohesive, and organic soils. Cohesive soils are characterized by very small particle size where surface chemical effects predominate. The particles do tend to stick together the result of water- particle interaction and attractive forces between particles. Cohesive soils are therefore both sticky and plastic. Cohesive soils (mostly clays, but also silty clays and clay-sand mixtures with clay being predominant) exhibit generally undesirable engineering properties compared with those of granular soils. Clayey soils cannot be separated by sieve analysis into size categories because no practical sieve can be made with openings so small; instead, particle sizes may be determined by observing settling velocities of the particles in a water mixture (Coduto, 1999).Clayey soils tend to have low shear strengths and to lose shear strength further upon wetting or other physical disturbances. They can be plastic and compressible, and they expand when wetted and shrink when dried. Some types expand and shrink greatly upon wetting and drying. Cohesive soils can creep (deform plastically) over time under constant load, especially when the shear stress is approaching its shear strength, making them prone to landslides. They develop large lateral pressures and have low permeability (Coduto, 1999).Particle sizes in soils can vary from over 100 mm to less than 0.001mm. In BSCS the sizes ranges detailed in Figure 2.1 are specified. The terms clay, silt, sand, gravel, cobbles and boulders are used to describe only the sizes of particles between specified limits (Craig, 2004).

Figure 2.1: Particle size range (Craig, 2004)

Clay SoilsSoils that consist of silt, sand and, or gravel are primarily the result of physical and mild chemical weathering processes and retain much of the chemical structure of their parent rocks. However, this is not the case with clay soils because they have experienced extensive chemical weathering and have been changed into a new material quite different from the parent rocks. As a result, the engineering properties and behaviour of clays also are quite different from other soils (Coduto, 1999). Clays are generally has particle sizes less than about 2. According to the British Soil Classification System (BSCS), clay soil comprising 35% to 100% fines where the clay particles predominate to produce cohesion, plasticity and low permeability. The characteristics of clay soil are shown at Table 2.1.Table 2.1: Characteristics of Clay Soil (Meschyan, 1995).CHARACTERISTICS OF CLAY SOIL

Specific Gravity2.55 2.75

Bulk Density (Mg /)1.50 2.15

Dry Density (Mg /)1.20 1.75

Void Ratio0.42 0.96

Liquid Limit (%)Over 25

Plastic Limit (%)Over 20

Effective cohesion (kPa)20 - 200

The properties of clay soil depend on the mineral composition of the particles, their shape and size, the type and strength of structural bonds, the structure, texture and interaction with water (Das, 2006). To construct on such soils, either pre-treatment or specially designed foundations can be used for low-cost construction to build houses and road infrastructures (Chan, 2006). It is therefore not deemed practical to be removed and replaced for construction works as this process is expensive and time-consuming. These applications require the knowledge of physical properties of soft clay and their implications on the usage of soft clay in the field. Clay according to the Unified Soil Classification System (USCS), are fine-grained soils with more than 50% by weight passing No. 200 US Standard Sieve (0.075mm) which have much larger surface areas than coarse-grained soils and responsible for the major physical and mechanical differences between coarse-grained soils.Batu Pahat Soft ClaySoft soils in the grounds of Universiti Tun Hussein Onn, are low in shear strength and bearing capacity, and suffer large settlements when subjected to loading(Chan, 2006). Based on the index properties of the soil, the soil can be categorized as CH (Inorganic Clays of High Plasticity) according to Unified Soil Classification System (Robani and Chan, 2009; Chan and Ibrahim, 2008). The physical properties of Batu Pahat soft clay at RECESS have been experimentally investigated by many researchers as shown in Table 2.2. A study carried by Chan and Ibrahim (2008), found that clay soil at RECESS, UTHM contained 10.8 % clay, 79.5 % silt and 10.7 % sand. They reported some physical properties of typical Batu Pahat soft clay at RECESS. Robani and Chan (2009) also conducted a study of Batu Pahat soft clay at RECESS test site, UTHM at a depth of 1.8 m. The sample was disturbed sample and the basic characteristics of the in-situ soft soil are reported with the average moisture content was about 84 %. They also identified that the clay soil at RECESS, UTHM contained 10.23 % clay, 89.2% silt and 0.57 % sand. Ho and Chan (2011) also studied the correlation of mechanical properties of Batu Pahat soft clay and the effect towards the surrounding soft clay when the soft clay is being stabilized homogenously and in a columnar system. The mechanical properties examined included one-dimensional compressibility and undrained shear strength. They reported that the higher value of cement content, the greater is the enhancement of the yield stress and the decrease of compression index.Table 2.2: Physical properties of Batu Pahat soft clay (Chan and Ibrahim, 2008; Robani and Chan, 2009; Ho and Chan, 2011)

ParametersResearchers

Chan and Ibrahim (2008)Robani and Chan (2009)Ho and Chan (2011)

Bulk Density (Mg /)1.36--

Specific Gravity2.662.622.62

Plastic Limit (%)313232

Liquid Limit (%)776868

Plasticity Index (%)4436-

Moisture Content (%)-8485

The study indicated that Batu Pahat Soft Clay has high moisture content (Chan and Ibrahim, 2008; Robani and Chan, 2009; Ho and Chan, 2011), low shear strength, low permeability, high compressibility, shrinks when dried and expands when wetted (Chan 2006). As the moisture content increases a clayey or silty soil will become softer and stickier until it cannot retain its shape when it is described as being in a liquid state. If the moisture content is increased further then there is less and less interaction between the soil particles and slurry, and a suspension is formed. If the moisture content is decreased the soil becomes stiffer as shown in Table 2.3 until there is insufficient moisture to provide cohesiveness when the soils becomes friable and cracks or breaks up easily if remoulded.Table 2.3: Typical moisture contents (Barnes, 2000)Soil typeMoisture content %

Moist sand5-15

Wet sand15-25

Moist silt10-20

Wet silt20-30

Normally consolidated clay low plasticity20-40

Normally consolidated clay high plasticity50-90

Overconsolidated clay low plasticity10-20

Overconsolidated clay high plasticity20-40

Organic clay50-200

Extremely high plasticity clay100-200

Peat100->1000

setel

Chemical StabilizationChemical stabilization involves the blending of natural soils with chemical agents such as portland cement, lime and asphalt. These agents generally are potential binders and as such effectively bind together the soil aggregates to achieve properties binders and as such as improved load, carrying and stress, distributing characteristics, and the control of shrinkage and swell (Garber and Hoel, 2009). Chemical admixture always involves for the treatment of natural soil with some kind of chemical compound, which when added to the soil would result in chemical reaction (Bujang 2005). The chemical reaction modifies or enhances the physical and engineering properties of that soil such as volume stability and strength. However, chemical stabilized like cement, lime and bentonite has two folds effect on soil characteristics of fluctuation, the clay particles are electrically attracted and aggregated with each other. This results in an increase in the effective size of clay size aggregation and such aggregation converts clay into the mechanical equivalent of fine silt. Also, a strong chemical bonding force develops between the individual particles in such aggregation. The chemical bonding depends upon the type of stabilizer employed (Bujang 2005). The physical and mechanical properties of stabilized soils depend on several factors, mainly the properties of base material and the environmental aspects. The strength development of stabilized soil depends on many factors such as type and properties of soil, quantity and type of admixture, moisture content, mixing and compaction method, condition and curing time, temperature, soil minerals and used admixture. Stabilized clay is the end product of stabilization, a ground improvement technique where chemical substances known as binders or stabilizers are added in existing soft soil to increase its strength and reduce its compressibility (Schaefer et al. 1997; Lin and Wong 1999). Rafizul et al. (2012) , studied the geotechnical parameters of stabilized soil prepared in the laboratory by mixing cement, lime and bentonite at varying content. The effect of admixtures content on compressive strength (q u ), changes of liquid limit (W L ) with mixing water, variation of compaction parameters with admixture and organic content as well as develop a linear regression model using SPSS were highlighted by the author. The liquid limit of stabilized soil decreases with the increases of admixtures content, however, the stabilized soil for 100 % mixing of water had more liquid limit than that of stabilized soil of 50 % mixing of water. Moreover, the maximum dry density was increase, while the optimum water content decreases with the increasing of admixtures content. The computed compressive strength from the developed regression model was almost same as the laboratory measured value and the degree of accuracy was more accurate and reliable. The higher strength was obtained from stabilized soil that have been cured for 28 day compared with the 1, 3, 7 and 14 day cured samples, moreover cement stabilized soil depicts the highest compressive strength than that of lime and bentonite stabilized soil.

Cement StabilizationSoil strengthening is required in many land reclamation projects. The desired properties of the improved soil are increased strength, reduced compressibility, and appropriate permeability to solve stability, settlement, ground water, and other environmental-related problems. Soft clay formations, especially those with high in situ water contents, are susceptible to large settlements and possess low shear strength unless they are naturally cemented. Precompression of such deposits with geodrains can prevent this large settlement and thus enhance shear strength. But this mode of attacking the problem often requires more time than is practically available. An alternative to this is cementation of the soft clay with supplementary cementing materials such as lime and cement (Horpibulsuk, et al., 2004). The principle mechanism of ground improvement is done by forming chemical bonds between the soil particles. When the soil particles are bonded, it will be strengthened and become more stable physically and mechanically. Soft clay, when mixed with cement, will be stabilized because cement and water react to form cementitious calcium silicate and aluminate hydrates, which bind the soil particles together (Gueddouda et al., 2011). The study of the treatment of clays using several methods of stabilization (addition of NaCl salt, lime, cement, and association lime+ cement, and association lime + salt) show that for certain combinations the reduction rate in swelling potential more than 90% (Gueddouda et al.,2011).Ordinary Portland Cement (OPC) is one of the most successfully used soil stabilization. It will reduce soil plasticity with resultant effects on swelling and similar behavior (Marian & Raymond, 1999). They found that the improvement of soil characteristics depended on the chemical components of cementing agent and the properties of the soil. OPC and soil mixed at the proper moisture content has been used increasingly in recent years to stabilize soils in special situations. The hardening process of cement stabilized soils happens immediately upon mixing soil with cement slurry. The hardening agent produces the hydrated calcium silicates, hydrated calcium aluminates, and calcium hydroxide and forms hardened cement bodies.

In other study, Saadeldin et al. (2006), evaluated the performance of a road embankment constructed on cement-stabilized soft clay (CSC). The undrained shear strength of the soft clay was experimentally determined before and after stabilization with cement. The results of the experimental work were used to simulate the behavior of the foundation soil under the road embankment using a 2-D finite element model. The foundation soil consisted of two layers: CSC having a variable thickness ranging from 1 to 5m, followed by soft clay layer extending to 15m below ground surface. The performance of the embankment founded on CSC was compared to that obtained if the CSC was replaced with compacted sand fill. Cement stabilization enhanced the performance of the embankment with respect to safety against shear failure more than sand soil replacement. It also found, the unconfined compressive strength of cement-stabilized soft clay increased as the cement content increased. The unconfined compressive strength increased as the curing time increased up to about 28 days, after which the compressive strength practically stabilized.The physical properties of soil cement depend on the nature of soil treated, the type and amount of cement utilized, the placement and cure conditions adopted (Purushothama, 2005). He suggested that soil-cement content varying from 5% to 20% for satisfactory stabilization. For clays, cement content may range from 3 to 16% by dry weight of soil, depending on the type of soil and properties required. Generally as the clay content of soil increases, so does the quantity of cement required (Bell, 1996).setelSoil compactionThe compaction effort test shows that in order to obtain maximum strength and durability of clay soil, it is necessary to carefully establish the kind and quantity of additive used, the optimum moisture content to maximize the compaction effort and the achievable dry density. Using the compaction results, the different mix combinations were moulded as near to their optimum moisture contents as possible, thus enhances engineering properties and optimizing compaction effort. The effect of stabilizers on maximum dry density and optimum moisture content was studied by Hossain and Mol (2011). They found that maximum dry density decreases and the optimum moisture content increases with the increase of volcanic ash, lime, fly ash and rice husk ash stabilized clayey soils.Shear StrengthThe shear strength of a soil mass is the internal resistance per unit area that the soil mass can offer to resists failure and sliding along any plane inside it. For most soil mechanics problems, it is sufficient to approximate the shear stress on the failure plane as a linear function of the normal stress. The shear strength of a soil in any direction is the maximum shear stress that can be applied to the soil structure in that direction. When this maximum has been reached, the soil is regarded as having failed, the strength of the soil having been fully mobilized (Murthy, 2008). Stabilization of a soil is commonly assessed in terms of strength gain over a certain period of time (cure). Strength gain is typically assessed by unconfined compressive strength (UCS) shear strength testing (Holt, 2010). According to Murthy (2008), UCS is preferred for clays because that UCS strength can exist only for clay by virtue of their cohesion component of the shear strength.

Factors Affecting the Strength of Stabilized SoilPresence of organic matters, sulphates, sulphides and carbon dioxide in the stabilized soils may contribute to undesirable strength of stabilized materials (Netterberg and Paige- Green, 1984, Sherwood, 1993).I. Organic MatterIn many cases, the top layers of most soil constitute large amount of organic matters. However, in well drained soils organic matter may extend to a depth of 1.5 m (Sherwood, 1993). Soil organic matters react with hydration product e.g. calcium hydroxide (Ca(OH)2) resulting into low pH value. The resulting low pH value may retard the hydration process and affect the hardening of stabilized soils making it difficult or impossible to compact.II. SulphatesThe use of calcium-based stabilizer in sulphate-rich soils causes the stabilized sulphate rich soil in the presence of excess moisture to react and form calcium sulphoaluminate (ettringite) and or thamausite, the product which occupy a greater volume than the combined volume of reactants. However, excess water to one initially present during the time of mixing may be required to dissolve sulphate in order to allow the reaction to proceed (Little and Nair, 2009; Sherwood, 1993).III. SulphidesIn many of waste materials and industrial by-product, sulphides in form of iron pyrites (FeS2 ) may be present. Oxidation of FeS2 will produce sulphuric acid, which in the presence of calcium carbonate, may react to form gypsum (hydrated calcium sulphate) according to the reactions (i) and (ii) below:i. 2FeS2 + 2H2O +7O2 = 2FeSO4 + 2H2SO4ii. CaCO3 + H2SO4 + H2O = CaSO4 .2H2O + CO2The hydrated sulphate so formed, and in the presence of excess water may attack the stabilized material in a similar way as sulphate (Sherwood, 1993). Even so, gypsum can also be found in natural soil (Little and Nair, 2009).IV. CompactionIn practice, the effect of addition of binder to the density of soil is of significant importance. Stabilized mixture has lower maximum dry density than that of unstabilized soil for a given degree of compaction. The optimum moisture content increases with increasing binders (Sherwood, 1993). In cement stabilized soils, hydration process takes place immediately after cement comes into contact with water. This process involves hardening of soil mix which means that it is necessary to compact the soil mix as soon as possible. Any delay in compaction may result in hardening of stabilized soil mass and therefore extra compaction effort may be required to bring the same effect. That may lead to serious bond breakage and hence loss of strength. Stabilized clay soils are more likely to be affected than other soils (Figure 1) due to alteration of plasticity properties of clays (Sherwood, 1993). In contrary to cement, delay in compaction for lime-stabilized soils may have some advantages. Lime stabilized soil require mellowing period to allow lime to diffuse through the soil thus producing maximum effects on plasticity. After this period, lime stabilized soil may be remixed and given its final compaction resulting into remarkable strength than otherwise (Sherwood, 1993).

Figure 3: Dry density versus time elapsed since the end of mixing of two material stabilized with 10% cement (Sherwood, 1993)

V. Moisture ContentIn stabilized soils, enough moisture content is essential not only for hydration process to proceed but also for efficient compaction. Fully hydrated cement takes up about 20% of its own weight of water from the surrounding (Sherwood, 1993). For soils with great soil-water affinity (such as clay, peat and organic soils), the hydration process may be retarded due to insufficient moisture content, which will ultimately affect the final strength.